Usually, a resistive bolometric detector measures the power of an incident radiation in the infrared range. For this purpose, it comprises an absorbing resistive bolometric element, which converts the light flow into a heat flow, which generates a temperature rise of said element with respect to a reference temperature. This temperature increase then induces a variation of the electric resistance of the absorbing element, thus causing voltage or current variations thereacross. Such electric variations form the signal delivered by the sensor.
However, the temperature of the absorbing element is usually highly dependent on its environment, and especially on the temperature of the substrate which comprises the electronic read circuit. To desensitize as much as possible the absorbing element from its environment, and thus increase the detector sensitivity, the absorbing element is generally thermally insulated from the substrate.
FIG. 1 is a simplified perspective view of an elementary resistive bolometric detector 10 of the state of the art illustrating this thermal insulation principle. Such an elementary detector, appearing in the described example in the form of a suspended membrane, conventionally belongs to a one- or two-dimensional array of elementary detectors.
Detector 10 comprises a thin membrane 12 absorbing the incident radiation, suspended above a substrate-support 14 via two conductive anchoring nails 16, having said membrane attached thereto by two thermal insulation arms 18. Membrane 12 usually comprises a layer of electric insulator, such as for example SiO2, SiO, SiN, ZnS or the like, which ensures the mechanical stiffness of membrane 12, as well as an electric metal interconnection layer deposited on the insulator layer.
A thin layer 20 of resistive thermometric material is further deposited at the center of membrane 12 on the metal interconnection layer, especially a layer made of a semiconductor material, such as strongly or weakly resistive polysilicon or amorphous p- or n-type silicon, or a vanadium oxide (V2O5, VO2) formed in a semiconductor phase.
Finally, the substrate-support 14 comprises an electronic circuit integrated on a silicon wafer, usually known as a “read circuit”. The read circuit comprises, on the one hand, the excitation and read elements of thermometric element 20 and, on the other hand, the multiplexing components which enable to serialize the signals originating from the different thermometric elements present in the array detector.
In operation, membrane 12 heats up under the effect of an incident electromagnetic radiation and the generated thermal power is transmitted to thermometric material layer 20. Periodically, the read circuit arranged in substrate 14 polarizes membrane 12 by submitting nails 16 to a polarization voltage, and collects the current flowing through thermometric element 20 to deduce therefrom a variation of its resistance, and thus the incident radiation having caused said variation.
For brevity, since the arrangement and the operation of such a detector are conventional, it will not be explained in further detail. It should however be noted that membrane 12 fulfils, in addition to the thermal insulation function, three main functions: an antenna function to receive the radiation, a function of conversion of the received electromagnetic power into thermal power, and a function of thermometric measurement of the generated thermal power. Since it is used as an antenna, membrane 12 has dimensions which are accordingly selected to be of the same order of magnitude as the wavelength of the radiation intended to be measured.
Now, in the terahertz range, wavelengths may reach one millimeter, which thus requires a membrane of the same order of magnitude. However, for such dimensions, the thermal mass, the mechanical hold, and the radiation loss of the membrane are such a problem that, in the end, they adversely affect the detector efficiency.
This is why, for such a frequency range, the radiation receive function is decoupled from the other functions. The receive function is thus ensured by a planar antenna, and the function of conversion of the electromagnetic power into thermal power is ensured by the resistive load of the antenna. The load dimensions conventionally fulfill the impedance matching conditions, which depend on the geometry of the antenna and on the nature of the layers supporting it, to obtain an optimal conversion. The resistive load is further in thermal contact with a thermometric element for the measurement of the generated thermal power. The assembly then forms a bolometer with an antenna.
Document US-A-2006/0231761 describes a “direct resistive coupling” bolometer with an antenna 30 operating in the millimetric range, for example, in the range from 1 to 10 THz, and provided with a bowtie antenna 32, simplified perspective and cross-section views thereof being respectively illustrated in FIGS. 2 and 3. The bolometer is made in the form of a membrane 34 suspended above a substrate-support 36 via two conductive anchoring nails or “holding arms” 38, to which it is attached by two thermal insulation arms 40. Membrane 34 comprises bowtie antenna 32, a resistive load 42 in the same plane as bowtie antenna 32 and in contact therewith, as well as a thermometric element 44 formed on and/or under resistive load 42 and in contact therewith. Resistive load 42 has a sheet resistance optimized to obtain a maximum resistive coupling with the antenna, especially a sheet resistance ranging between 100Ω and 200Ω. An explanation of the bowtie antenna can for example be found in R. PEREZ's thesis, which can be consulted on the following site: http://www.unilim.fr/theses/2005/sciences/2005limo0053/perez_r.pdf.
A simple antenna cannot absorb all the incident power and part of the incident power is transmitted to the rear of the antenna. More specifically, part of the absorbed incident power is reemitted to the rear of the antenna. To enhance the general absorption, a reflective assembly 46 is provided in substrate-support 36 above read circuit 48 comprising on the one hand the excitation and read elements of thermometric element 44, and on the other hand the multiplexing components which enable to serialize the signals originating from the different thermometric elements present in the array detector.
Reflective assembly 46 comprises a reflective layer 50 on which is deposited a dielectric material layer 52 satisfying the following relation:e=λ/(4n)  (1)where λ is a wavelength for which, and around which a resonance phenomenon is desired, e is the thickness of layer 52, and n=√{square root over (∈)}, with ∈ standing for the dielectric permittivity of the material forming layer 52. A quarter-wave cavity is thus obtained.
In such a configuration, the thermometric element is independent from the antenna, and its size then no longer depends on the incident wavelength but on factors determining the intrinsic performance of the detector (sensitivity, signal-to-noise ratio, etc. . . . ), in accordance with the requirements of the targeted application, for example, active imaging or passive imaging.
Further, in most cases, the incident electromagnetic radiation is not polarized, whereby its reception by a single antenna does not enable to capture the entire electromagnetic power. However, a non-polarized radiation may be considered as resulting from the superposition of two components linearly polarized in two orthogonal directions, each of these components transporting half the power of the wave. As known per se, an efficient way to capture an incident electromagnetic radiation is to use two crossed bowtie antennas.
However, the use of two crossed bowtie antennas suspended on a membrane induces a substantial decrease of the thermal resistance of the membrane since one of the bowtie antennas inevitably crosses the thermal insulation arms.
Referring to the simplified top and cross-section views of FIGS. 4 and 5, document U.S. Pat. No. 6,329,655 describes a bolometer 60 with an antenna, operating in the millimetric range and provided with two crossed bowtie antennas 62, 64, which overcomes this problem.
The principle of bolometer 60 is based on the capacitive coupling achieved between antennas 62, 64, arranged on a substrate-support 66, and a resistive load 68, arranged in a suspended membrane 70 and having a thermometric element 72 placed thereon.
Resistive load 68, which takes the form of a square layer arranged vertically above the center of antennas 62, 64, has a surface opposite thereto and thus forms a capacitance with antennas 62, 64. The radiation captured by antennas 62, 64 is thus transmitted to load 68 by capacitive coupling.
The resistance of load 68 and the value of the capacitance that it forms with antennas 62, 64 are selected to establish a high capacitive coupling, advantageously to fulfill at best the following relation, which achieves an optimal impedance matching between antennas 62, 64 and resistive load 68, and thus the optimal capacitive coupling:
                              (                      1                          π              ·              f              ·              C                                )                2            +                        (                      R            ·            C                    )                2              ≈      100    ⁢                  ⁢    Ω  where f is the radiation frequency, C is the value of the capacitance formed between antennas 62, 64 and resistive load 68, and R is the value of the resistance of resistive load 68. The characteristics of the capacitive coupling thus especially set, via the value of capacitance C, height ebolometer—cavity of the cavity separating membrane 70 from substrate-support 66.
Finally, for the previously-discussed reasons, to enhance the general absorption of the radiation, a reflective assembly 72 is provided in substrate-support 66 above read circuit 74, comprising, on the one hand, the excitation and read elements of thermometric element 68, and on the other hand the multiplexing components which enable to serialize the signals originating from the different thermometric elements present in the array detector.
Reflective assembly 72 comprises a reflective layer 76 having a quarter-wave cavity formed by a dielectric material layer 78 deposited thereon, fulfilling previously-described relation e=λ/(4n). Together, reflective assembly 72 and antennas 62, 64 form a resonant cavity.
The principle of a resonant cavity for the thermometric detection is described in relation with FIG. 6. As known per se, a resonant cavity is formed by establishing a constructive interference between a monochromatic radiation I, of wavelength λ, incident on an absorbing element, especially antennas, and radiation R reflected by the reflective assembly formed of a metallic reflective layer 82, having material layer 80 deposited thereon, dielectric permittivity ∈ and thickness e of layer 80 being selected according to relation (1) to adjust the resonance to the desired wavelength. The absorbing element is formed on layer 80, as illustrated by element 90 and/or suspended thereabove, as illustrated by element 92.
Thereby, the phase shift between incident radiation I and reflected radiation R is zero at distance e from reflective layer 82, so that a constructive interference is obtained at this distance, which provides an optimal absorption of the power of the incident electromagnetic field. As previously mentioned, layer 80 is currently called a “quarter wave cavity” due to relation e=λ/(4n).
An optimal arrangement of a suspended bolometric membrane and of a quarter-wave cavity is achieved when layer 80 preferentially is a layer of vacuum, or even of a gas of low thermal conductivity, and the bolometric membrane is placed at distance e from reflective layer 82, which is then deposited on a substrate 84 comprising the read circuit of the membrane. In such a case, the membrane is both insulated from substrate 84 and placed at the location where the electromagnetic field power absorption is maximum.
This type of layout is for example currently used for infrared thermometric detection, for which thickness e ranges between 2 micrometers and 5 micrometers, which distance allows a mass production of suspended membranes having a high-quality mechanical hold. Indeed, as known per se, to form a membrane suspended above a support, a sacrificial layer is first deposited on the reflective layer, after which the membrane is constructed on the sacrificial layer. Once the membrane has been formed, the sacrificial layer is removed so that the membrane is suspended, generally in vacuum, above the support. By providing the support with a reflective layer and by selecting a sacrificial layer having the desired thickness e, a quarter-wave cavity is thus obtained with the membrane placed at thickness e from the reflective layer.
This layout is however difficult to achieve for a terahertz detection. Indeed, for such a wavelength range, thickness e is greater than 10 micrometers, or even greater than 30 micrometers. First, support pads having such a height do not provide a high-quality mechanical hold of the suspended membrane. Second, problems of separation of the sacrificial layer can be observed for such thicknesses due to the residual stress generated during the deposition that it undergoes on manufacturing of the membrane, which stress increases as the sacrificial layer thickness increases.
This is why a state-of-the-art reflective assembly 46, 72, such as for example illustrated in FIGS. 3 and 5, is made of a solid material 52, 78, and bolometric membrane 34, 70 is suspended above reflective assembly 46, 72 by a few micrometers for its thermal insulation, by means of thermal insulation arms having an appropriate thickness, width, and length. This space above the reflective assembly is created by the deposition of a sacrificial layer having a thickness which must remain compatible with its mechanical stability during the microbolometer manufacturing method and further be compatible with the forming of contacts between the suspended bolometric membrane and the substrate comprising reflective assembly 46, 72.
This enables to have support pads of reasonable height and avoids sacrificial layer separation problems. The membrane suspension height is then no longer imposed by the thickness of the reflective assembly and can thus be much smaller. It can thus be noted that bolometric membrane 34, 70 is no longer optimally placed with respect to reflective assembly 46, 72, but at a distance of a few micrometers thereabove, taking advantage of the fact that there still is a phenomenon of optimization of the electromagnetic field power at this distance, as illustrated in the diagram of the left-hand portion of FIG. 6. This drawing illustrates the intensity of the electromagnetic field, which decreases as the distance from reflective layer 82 increases. The state-of-the-art layout for terahertz detection thus is a compromise between the technical feasibility of a membrane suspended above a reflective assembly for which the quality factor is desired to be optimized.
Such a layout however poses a number of specific problems.
First a solid layer 52, 78 of large thickness requires contact recoveries between the read circuit and the bolometric membrane, which are difficult to form, especially causing efficiency drops. For example, for a detector intended to detect a radiation around 700 GHz, the thickness of reflective assembly 46, 72 comprising a layer of silicon oxide (one of the materials most currently used in microelectronics) is approximately 33 micrometers. Now, to manufacture at a large scale a layer of such a thickness crossed by connection vias, at least three deposition steps, each associated with the forming of a portion of the vias, have to be implemented.
Finally, layer 52, 78 is deposited on a metal layer to which it may not adhere sufficiently to avoid any risk of separation during subsequent manufacturing steps.
To decrease the thickness of layer 52, 78, a dielectric of high dielectric permittivity may be used to comply with relation (1), and thus at least partially solve these problems. However, as the dielectric permittivity increases, electromagnetic phenomena appear, which may adversely affect the operation of bowties antennas, especially causing an increase of the coupling between antennas, which alters the quality of the detection. The selection of the reflective assembly thus generally results from a compromise between several antagonistic phenomena.